This invention relates to the separation of rare earths and more specifically to an improvement in the method for separation of the various earths and yttrium from each other by solvent extraction. The separation of the rare earths and yttrium from each other in order to obtain them in their pure state has been a difficult process in the past because the chemical and physical properties of the rare earths are so similar. Since yttrium is found and extracted along with the rare earths, this element is included hereinafter in the term "rare earths". One early process which permitted separation of the rare earths by solvent extraction is described in U.S. Pat. No. 2,955,913 which issued Oct. 11, 1960. In this process, it is disclosed that, when a concentrated mineral acid solution containing a plurality of rare earths is contacted with water-immiscible alkyl phosphates such as tributyl phosphate, extraction occurs for various rare earths in various degrees, the extraction increasing with increasing atomic number and increasing atomic weight, the yttrium extracting as if it had an atomic number between 66 and 67. However, the extractability values of the various rare earths in this process are rather close together so that many extraction and back-extraction cycles are required to achieve good separation between the various rare earths.
An improvement in the aforementioned process is described in U.S. Pat. No. 3,110,556, Nov. 12, 1963, in which a dialkyl phosphoric acid was shown to have greater separation extractability values for the rare earths and yttrium. The dialkyl phosphoric acid which showed the greatest promise and which has attained widespread acceptance for separation of the rare earths is di(2-ethylhexyl) phosphoric acid (referred to hereinafter as HDEHP). However, one major limitation with the use of HDEHP as an extractant for the rare earths and yttrium is gelation or the formation of insoluble polymers at approximately 50% saturation. The gelation which takes place has the effect of preventing the separation of the aqueous and organic phases, thus making the process impossible to operate. Thus gelation of the HDEHP necessitates that the concentration of rare earths in the aqueous mineral acid feed solution be severely limited to only a fraction of what they might be otherwise, reducing the efficiency of the separation process, lowering the throughput and consequently increasing the total cost of the rare earths recovered.
Another difficulty attendant with the use of HDEHP is that it has such a strong affinity for the higher atomic number rare earths that unless only a relatively low concentration of the extractant is used, it is difficult or impossible to re-extract or strip some of the elements from the solution.